This disclosure relates to a hydraulic system for driving a vibratory mechanism of a compaction roller. The hydraulic system comprising at least one hydraulic motor connectable to vibratory mechanism and a hydraulic pump fluidly connected to the at least one hydraulic motor and arranged for supplying pressurised hydraulic fluid to the at least one hydraulic motor. The disclosure also relates to a corresponding method for controlling a vibrator mechanism of a compaction roller. The hydraulic system may be installed on a compaction machine comprising a single, dual or more compaction rollers.
Compaction machines are used for compacting the ground on construction work sites to accomplish a smooth and flat ground surface, in particular in earthwork and road construction. The ground surface may comprise soil, gravel, asphalt and the like. The compaction machine comprises at least one substantially cylindrical compaction roller that presses the soil flat. The compaction machine relies partly on its static mass and partly on a dynamic compacting force to generate a high compacting force at the contact surface between the compaction roller and soil surface. The dynamic compacting force is generated by operating a vibratory mechanism associated with the at least one compaction roller. The vibratory mechanism comprises at least one weight that is eccentrically offset from a rolling axis of the compaction roller, and upon rotation of the weight by means of vibration drive a centrifugal force is generated due to the eccentricity and a relatively high inertia, thereby producing the dynamic compacting force.
Compaction machines on the job site typically drive forward and backward in a sequence of for example 30 seconds. During each change of direction the vibration drive is preferably switched off for avoiding detrimental effects on the compacted surface. The eccentric mass has high inertia which is accelerated and decelerated each time the machine reverses the direction of travel. To avoid interference with natural frequencies of the structure of the machine and to improve the productivity, the vibration drive need to be accelerated and stopped quickly, preferably in less than 10 seconds and more preferably in less than 5 seconds. The vibration drive is typically of hydrostatic nature. The needed torque to accelerate the inertia is inverse proportional to the launch time. Therefore the power of the eccentrics′ hydraulic pumps and motors is designed for this start/stop activity. During the steady run the needed torque (rotary power) is typically significantly less than half of the start-up torque.
In traditional hydraulic systems for vibration drives comprising a fixed displacement pump a relatively large amount of energy is lost in throttling; losses caused by the difference in supplied flow by the pump and consumed flow by the motor. The flow difference, which gradually decreases with increased motor speed, is guided back to the tank via a pressure relief valve. Document WO2011095200 describes a solution for reducing the level of energy losses without necessarily compromising on acceleration level. This solution comprises a hydraulic accumulator and valve assembly for storing the kinetic energy of the eccentrics during the deceleration and for reusing the energy to accelerate them again. There is however still room for improvements with respect to fuel efficiency and cost-efficiency of the compaction machine.
It is desirable to provide a hydraulic system that provides improved fuel efficiency of the eccentric drives and enables use of as power source having less maximal output power while maintaining a quick acceleration phase of the eccentric drive.
The disclosure concerns, according to a first aspect, a hydraulic system for driving a vibratory mechanism of a compaction roller, wherein the hydraulic system comprising at least one hydraulic motor connectable to vibratory mechanism and a first hydraulic pump fluidly connected to the at least one hydraulic motor and arranged for supplying pressurised hydraulic fluid to the at least one hydraulic motor.
The disclosure, according to the first aspect, is characterized in that the hydraulic system further comprises a second hydraulic pump fluidly connected to the at least one hydraulic motor and arranged for supplying pressurised hydraulic fluid to the at least one hydraulic motor.
In conventional hydraulic systems for driving a vibratory system a power source, typically a diesel engine drives a single fixed displacement hydraulic pump for delivering hydraulic fluid to a hydraulic motor via a control valve assembly. Relief pressure valves provide a safe and proper operation of the hydraulic system by eliminating excessive and potentially damaging pressure build-up in the hydraulic system. The single fixed displacement pump has to have sufficient flow capacity to accelerate the hydraulic motor and the associated vibratory mechanism to a nominal speed. During the acceleration period of the vibratory mechanism the single fixed displacement hydraulic pump constantly delivers high flow volumes. Due to the constant flow of the pump approximately half of this energy will be dissipated at the pressure relief valves, because the hydraulic motor at the eccentrics speed up continuously and the flow through the hydraulic pump increases from zero to full pump flow. The pressure relief valve, which influences the acceleration level of the hydraulic motor, is selected to avoid any damages of the hydraulic system due to excessive pressure. The single fixed displacement pump system will consequently require a relatively high power output from the engine during the complete acceleration time.
The hydraulic system according to a first aspect comprises a first and a second hydraulic pump fluidly connected to the at least one hydraulic motor and both are arranged for supplying pressurised hydraulic fluid to the hydraulic motor. This arrangement enables, by proper dimensioning and operation of the first and second hydraulic pumps, improved fuel efficiency of the eccentric drives while maintaining a quick acceleration phase of the eccentric drive. These advantageous aspects may for example be realised by supplying pressurised hydraulic fluid to the at least one hydraulic motor from only one of the first and second hydraulic pumps during a first part of a hydraulic motor acceleration phase, and to supply pressurised hydraulic fluid to the at least one hydraulic motor from both of the first and second hydraulic pumps during a second part of the hydraulic motor acceleration phase. This arrangement has the advantage that each hydraulic pump may exhibit a smaller displacement compared with the displacement of the single fixed displacement pump according to the conventional solution. Operation of a smaller displacement pump requires less engine power than operation of a larger displacement pump at the same engine speed during the acceleration phase because less flow, i.e. energy will be dissipated at the pressure relief valve. After a certain time period of operation of a single hydraulic pump also the second hydraulic pump is operated. The combined displacement of the first and second hydraulic pumps may be selected to correspond to the displacement of the conventional single pump design, such that the hydraulic motor may be accelerated to the desired speed.
According to farther aspect of the disclosure, the hydraulic, system further may comprise a hydraulic accumulator fluidly connected to the at least one hydraulic motor. Thereby at least part of the kinetic energy of the eccentric can during deceleration thereof be converted to hydraulic energy and temporarily stored in the hydraulic accumulator, and upon later acceleration of the eccentric the stored hydraulic energy can be used to accelerate the eccentric. Use of the accumulator enables significant reduction or even a complete elimination of dissipation of energy at the relief valve, thereby reducing overall fuel consumption.
According to yet a further aspect of the disclosure, one of the first and second hydraulic pumps has a larger maximal displacement volume than the other of the first and second hydraulic pump. The two pumps in sum guaranty that the nominal speed of the hydraulic motor is achieved. The recovered amount of rotary energy of the eccentrics is always less than the energy needed to accelerate the eccentrics to the same speed again due to normal unavoidable energy losses associated with the energy conversion and friction in bearings etc. However, the required additional energy in form of additional fluid flow is relatively small since the energy loss is relatively small. If the additional energy is supplied after completed discharge of the accumulator the total fluid flow that must be supplied by the first and second pumps is relatively large since it corresponds to the flow at nominal motor speed. The supply pressure must also be relatively high to provide the required acceleration level. The current engine torque input equals current pump supply pressure times current total pump supply flow. Hence, the engine must be able to provide a relatively large peak output power during this short period to accelerate the hydraulic motor up to the nominal speed. Also the components of the power train, especially the engine and the pump need to be designed for this peak power.
However, if the additional fluid flow from the smaller pump is provided concurrently with the flow from the accumulator the deliver flow level must merely correspond to said energy loss occasioned by said energy conversion associated with the hydraulic accumulator during deceleration/acceleration. Consequently, by having a smaller displacement pump and a larger displacement pump, and by operating only the smaller displacement pump during the acceleration phase, i.e. as an acceleration pump, and by operating the larger displacement pump only upon having reached the nominal motor speed, i.e. a steady-state mode, the engine peak power can be significantly reduced. The smaller pump may be also be designed as a high pressure pump capable of deliver flow at the high pressure needed for sufficient acceleration level of the eccentrics. The larger pump however may be designed to deliver only the steady state pressure level of the running eccentrics, which pressure level is significantly lower than the acceleration pressure. The larger pump may thus be manufactured in less durable material and with lower demands with respect to tolerances, such that the cost of the larger pump may be reduced. Furthermore, because the swept volume of the smaller pump is relatively small, even at high pressure the required torque output from the engine shaft is relatively small. Due to the reduced requirement of peak power the installed engine size can be reduced with the effect of better fuel efficiency and easier installation in the machine. Furthermore, this solution also enables variability in the frequency of the vibration by operating the smaller and larger pumps together or by operating only the larger pump of the hydraulic system. Operation of only the larger pump provides a lower frequency mode and by operating both pumps simultaneously a higher frequency mode is provided, all without the need for any additional components for providing the two different vibration frequencies.
Once the eccentrics achieved their nominal speed the larger pump can be connected too. The smaller displacement pump of the first and second hydraulic pump has a displacement volume in the range of 10%-90% of the larger displacement pump, preferably in the range of 20%-70%, and more preferably in the range of 25%-50%. The actual relative size of the first and second pumps will be determined based on the actual system design including in particular the amount of energy conversion losses.
The disclosure also concerns a method for controlling a vibratory mechanism of a compaction roller according to the first aspect. The vibratory mechanism is mechanically connected to at least one hydraulic motor arranged to be supplied with pressurised hydraulic fluid from a first and a second hydraulic pump. The method comprising steps of
accelerating the hydraulic motor by supplying pressurised hydraulic fluid to the at least one hydraulic motor from only one of the first and second hydraulic pumps during a first part of a hydraulic, motor acceleration phase, and
accelerating the hydraulic motor by supplying pressurised hydraulic fluid to the at least one hydraulic motor from both of the first and second hydraulic pumps during a second part of the hydraulic motor acceleration phase. This method will exhibit advantages corresponding to the hydraulic system of the first aspect described above. A smaller and a larger hydraulic pump enable use of a more cost-efficient and simple components to reduce the energy consumption of the vibratory drive of a compaction machine and allow significant reduction in engine peak torque requirement. Also, the smaller displacement pump may be designed to withstand a higher operating, pressure than the larger hydraulic pump because the larger displacement pump may be arranged to be operated first upon having reached the nominal motor speed. At a stage where the acceleration phase associated with the smaller displacement pump has terminated and the steady-state has been reached, a less complex and less costly pump is considered sufficient.
Further advantages are achieved by implementing one or several of the features of the dependent claims.
According to a further aspect of the disclosure, one of the first and second hydraulic pumps is a variable displacement pump and the other of the first and second hydraulic pumps is a fixed displacement pump. This layout enables an infinite variability of the frequency in a certain range if needed for optimizing the compaction result with respect to the environmental material. It is beneficial to replace only the smaller displacement pump by a variable displacement pump and to keep the larger displacement pump for the basic steady state flow. The potential combination of high flow at low pressure and small variable flow at high pressure with these two pumps allow a low-cost variable frequency drive.
The disclosure further relates to a compaction machine comprising such a hydraulic system, a computer program comprising program code means for performing the steps of the described method, a computer readable medium carrying a computer program comprising program code means for performing the steps of the described method when said program product is run on a computer, and a control unit for controlling the described hydraulic system.